Field of the Invention
The invention relates to a method for preparing an R—Fe—B based sintered magnet, which belongs to the field of rare earth permanent magnet materials.
Description of the Related Art
Nd—Fe—B based sintered magnet has been widely applied because of its superb performance. The improvement of the magnetic remanence and the coercivity facilitate the fast increase of the motor market. However, a conventional method for improving the coercivity has to pay the price of sacrificing the magnetic remanence as well as investing a large amount of the heavy rare earth elements of Dy/Tb thereby resulting in a sharp increase of the production cost of the magnet. In order to decrease the amount of the heavy rare earth elements including Dy/Tb and obtain a high-temperature resistance, grain boundary diffusion and two-alloy method are mainly used to develop the magnet having a low Dy content and a high coercivity.
The grain boundary diffusion is conducted on the Dy and Tb on the magnetic surface to improve the coercivity of the sintered magnet and decrease the magnetic remanence. The grain boundary diffusion is capable of largely decreasing the use of the heavy rare earth elements and further significantly lowering the production cost of the magnet. But a main technology currently applied is the surface penetration of Dy which includes machining the magnet into a lamina, arranging Dy or Tb on the magnetic surface, and performing high temperature treatment to enable the heavy rare earth to cross the liquid phase of grain boundary and enter the internal magnet. The method has a strict requirement on the size of the magnet, and a thickness required by the process is ≦7 mm.
The binary alloy method is able to improve the concentration of Dy on the main phase surface, so that the binary alloy method is supposed to saves Dy. A main phase alloy in the binary alloy method is Nd2Fe14B, and a promoter alloy includes Dy and Tb. Sintered magnet prepared by mixing such the main phase alloy powder and the promoter alloy powder is able to segregate Dy on the main phase surface. If Dy segregation around the grain boundary is realized, the saturated magnetization of the sintered magnet prepared by the binary alloy method is much higher than the common sintered magnet. However, the sintering temperature in a common method is required to be higher than 1000° C. to yield a required density of the magnet, a large amount of the heavy rare earth elements at the temperature are diffused to the main phase, thereby being difficult to realizing the purpose of accumulation of heavy rare earth elements in the grain boundary zone. Therefore, the binary alloy method has the same effect as the conventional method that directly adds heavy rare earth elements during the melting stage.
Surface coating or burying methods for improving the magnet properties of R—Fe—B magnet are restricted by the size of the magnet, and a thickness of the magnet is only less than 7 mm; besides, during the coating or burying, a poor control on the thickness of the coated layer and the density easily results in a high defect rate during the bath production. While in the conventional binary alloy method, a large amount of the heavy rare earth elements enter the main phase during the heat treatment of the high temperature sintering process, so that the rich Dy and the rich Tb alloy do not function in the optimization of the microstructure of the grain boundary, and the improvement of the magnet property is not obvious.